1. Field of the Invention
The present invention relates to method for continuous production of a water-absorbent resin, and in particular, relates to method for production of a water-absorbent resin by continuous polymerization of a monomer contained in an aqueous solution. A water-absorbent resin obtained by a method of the present invention is effectively used as sanitary articles such as a disposable diaper and a sanitary napkin, a water retaining agent for soil, etc.
2. Description of Related Art
A water-absorbent resin is a resin with high water-absorption capacity and is adopted to wide applications making use of performance thereof. For example, a water-absorbent resin is applied to sanitary articles such as a disposable diaper and a sanitary napkin, a water retaining agent for soil, etc.
Such a water-absorbent resin (also called as a superabsorbent polymer (SAP) or a water-absorbent polymer) is described, for example, in Japanese Industrial Standards (JIS) K7223-1996 and already known. The known water-absorbent polymers include partially neutralized and crosslinked polyacrylic acid, hydrolysates of acrylic acid grafted starch, saponified products of vinyl acetate-acrylic acid ester copolymers, hydrolysates of acrylonitrile or acrylamide copolymers or crosslinked polymers thereof, crosslinked polymers of cationic monomers, etc.
The water-absorbent resin is used in a disposable diaper and a sanitary napkin or a water retaining agent for soil in relatively large amount. Therefore, it has huge influence upon cost competitiveness of a product made with a water-absorbent resin. Consequently, the development of cost reduction arts in the production of the water-absorbent resin has been desired.
To produce the water-absorbent resin by an aqueous solution polymerization, a monomer aqueous solution containing a monomer, a polymerization initiator and a crosslinking agent is prepared. Aqueous solution polymerization proceeds in the aqueous solution to obtain a gel-like hydropolymer. After disintegrating a hydropolymer, a particulate hydropolymer is dried to obtain a particulate base polymer, followed by surface crosslinking of the base polymer to obtain a water-absorbent resin.
In such processes, the huge amount of energy is required to dry a hydropolymer. Production methods are disclosed to produce a hydropolymer with low water content by using an aqueous solution with high monomer concentration (See, for example, U.S. Pat. No. 4,552,938 and U.S. Pat. No. 4,985,518). However, a water-absorbent resin produced by such disclosed methods has a problem on the performance to be solved such as the resultant water-absorbent resins have high extractable content for their water-absorption capacities. A reason for that is not clear, but it is considered that a high molecular weight component formed in early polymerization stage under high concentration may exert a bad influence upon performance.
Therefore, to enhance performance of a water-absorbent resin, polymerization arts was developed by using an aqueous solution with low monomer concentration by controlling the temperature at not higher than a certain specified level (See, for example, U.S. Pat. No. 5,380,808 and U.S. Pat. No. 6,174,978). By adopting a mild polymerization condition, it is possible to obtain a water-absorbent resin with high performance. However, due to the generation of the heat of polymerization during the polymerization reaction, heat removal is required from a polymerization reaction solution. The large heat quantity is also necessary to dry a hydropolymer with high water content. That is, arts disclosed in U.S. Pat. No. 5,380,808 and U.S. Pat. No. 6,174,978 still have a problem on production cost, although they contribute to improvement of performance of a water-absorbent resin.
As arts satisfying both production cost and quality, a polymerization method has been disclosed wherein an aqueous solution containing acrylic acid or a sodium salt thereof in high concentration is polymerized at a high initial temperature, while vaporizing water (See U.S. published application 2002-40095). By using such a method, the cost for heat removal can be reduced because heat removal during the polymerization is not required. The cost necessary for drying can also be reduced because the water content in the hydropolymer formed is low. Furthermore, the produced water-absorbent resin has high performance.
Various arts have been disclosed on a production method for a water-absorbent resin like these. However, desire for the development of an effective production method of water-absorbent resin with high performance is still present.